1. Field of the Invention
The present invention relates to a rubber composition for an elastic member and an elastic member using the same. More particularly, the present invention is concerned with a rubber composition capable of affording an elastic member which has a low hardness and also excellent durability, and which is well suited for use in a variety of part items that are installed in an image formation apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, including copying machinery, printers, facsimile apparatuses and the like, in particular a developing roller; and an elastic member which is composed of the rubber composition, is imparted with the characteristics as mentioned above, and serves in an image formation apparatus.
2. Description of the Related Arts
With regard to an image formation apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, including copying machinery, printers, facsimile apparatuses or the like, an elastic member comprising a rubber composition has heretofore been employed in the form of a roller, blade, belt or the like, and played a variety of roles in a process, including electrification, development, transfer, toner layer regulation, cleaning, fixing, paper supply and paper transport.
Although it is said that an elastic member which comprises a rubber composition and is used in any of the above-mentioned processes is desired to have a low hardness from the viewpoint of performance, it has been impossible to realize such a low hardness because of several reasons except for the case of using a foam. The reason for a low hardness being desired is its capability of extending the allowable range of design in each process. For instance, however, a foam is difficult to use in a developing roller which supplies a toner onto a photosensitive body from the viewpoints of its durability, fineness of images, its reliability and the like. Likewise, a foam is difficult to use in a paper feed roller which transports recording paper in view of its wear resistance.
In addition, it is desired that the developing roller has a low hardness because of such reasons that toner transport is made easy by favorable contact with a photosensitive body, toner damage is lessened and the like and at the same time, the roller is required to have durability against wear caused by friction with a toner.
With regard to an electrophotographic developing apparatus such as copying machinery, printers, etc., there is previously known a pressurized developing method as an image formation method which comprises supplying a unary toner (developer) to a latent image preserving body such as a photosensitive body which preserves an electrostatic latent image, and visualizing the latent image by allowing the toner to adhere to the latent image (refer to U.S. Pat. Nos. 3,152,012 and 3,731,146).
The pressurized developing method carries out the image formation by bringing a developing roller that supports a toner into contact with a latent image preserving body (photosensitive body) preserving an electrostatic latent image, and allowing the toner to adhere to the latent image on the surface of the afore-said latent image preserving body, whereby the developing roller is required to be constituted of an electroconductive elastic body having both electroconductivity and elasticity.
Specifically in the foregoing pressurized developing method the constitution is such that, for instance, as illustrated in FIG. 1, a developing roller 1 is placed between a toner application roller 5 for toner supplying and a latent image preserving body 6 (photosensitive body) preserving an electrostatic latent image; the developing roller 1, the latent image preserving body 6 (photosensitive body) and the toner application roller 5 rotate each in the direction of the arrow in FIG. 1, thereby a toner 7 is supplied onto the surface of the developing roller 1 with the toner application roller 5, and is arranged into a uniform thin film by a layer forming blade 8; the developing roller 1 rotates in the state that the toner 7 is so arranged, while being in contact with the latent image preserving body 6 (photosensitive body) ; and the toner thus formed into a thin film is allowed to adhere to an latent image on the latent image preserving body 6 from the developing roller 1, whereby the aforesaid latent image is visualized. Symbol 9 indicates a transfer portion, where a toner image is transferred to a recording medium such as paper. Symbol 10 indicates a cleaning portion, where the cleaning blade 11 removes the toner which remains after the transfer on the surface of the latent image preserving body 6.
In such developing apparatus by means of the pressurized developing method as mentioned above, the developing roller 1 is obliged to rotate, while maintaining the state of close contact with the latent image preserving body 6. For this reason, the constitution of the developing roller 1 is such that as illustrated on the schematic cross-section of FIG. 2, a shaft 2 consisting of an electroconductive material such as a metal is equipped on its outside periphery with an electroconductive elastic layer 3 which is composed of an electroconductive elastic body which is imparted with electroconductivity by blending an electroconductivity imparting agent in elastic rubber such as silicone rubber, acrylonitrile butadiene rubber, ethylene propylene rubber and polyurethane rubber or foam thereof. In addition, a coating layer 4 composed of a resin or the like is installed on the surface of the electroconductive elastic layer 3 for the purpose of controlling electrostatic property and adhesivity for the toner 7, of controlling the force of friction between the latent image preserving body 6 and the layer forming blade 8, or of preventing fouling of the latent image preserving body 6 due to the elastic body.
In the case of performing the development of electrostatic latent images by using the developing roller such as the above through the pressurized developing method, the end surface of the developing roller is worn by the friction with the toner and the like that are left and accumulated on the end portion of the developing roller without serving for the development. This wear becomes the cause for generating a defective image and further for deteriorating the durability of the developing roller. In the case of performing the development of electrostatic latent images through the pressurized developing method, when the hardness of the surface of the developing roller is increased in order to enhance the wear resistance of the roller, the area of contact between the roller and the latent image preserving body such as a photosensitive body is decreased, thus resulting in failure to carry out favorable development as the case may be. In addition, an excessively high hardness of the surface of the developing roller often causes a damage to the latent image preserving body. What is more, an excessively high hardness of the developing roller itself causes a damage to a developer as the case may be because of an overload applied thereto between the roller and a layer regulating blade which is in butt contact with the roller.
Under such circumstances, a general object of the present invention is to provide a rubber composition capable of affording an elastic member which has a low hardness and excellent durability as well, and which is well suited for use in a variety of part items that are installed in an image formation apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, including copying machinery, printers, facsimile apparatuses and the like, in particular a developing roller; and an elastic member for an image formation apparatus which member is imparted with the characteristics as mentioned above by using the rubber-composition.
Another object of the present invention is to provide a developing roller which is capable of enhancing wear resistance for a developer without bringing about a high hardness of the roller, and a developing apparatus equipped with the developing roller.
Further objects of the present invention will be made obvious from the content of the specification hereinafter disclosed.
In such circumstances, intensive research and development were accumulated by the present inventors in order to solve the problems and thus achieve the above-mentioned objects. As a result, it has been found that the objects are achievable by a rubber composition comprising butadiene rubber and/or isoprene rubber in liquid form and silicone rubber each at a specific proportion.
In addition, it has been found that a developing roller which is enhanced in wear resistance for a developer is obtainable by forming an electroconductive elastic layer comprising the foregoing rubber composition on the outside periphery of a shaft having good electroconductivity without bringing about a high hardness of the roller.
That is to say, the present invention provides a rubber composition for an elastic member to be used in an image formation apparatus which composition comprises (A) butadiene rubber and/or (B) isoprene rubber in the form of liquid and (C) silicone rubber at a {(A)+(B)}/(C) ratio by weight in the range of 97/3 to 5/95.
Moreover, the present invention provides an elastic member for an image formation apparatus which member comprises the above-mentioned rubber composition for an elastic member.
Further, the present invention provides a developing roller which comprises a shaft having good electroconductivity and an electroconductive elastic layer formed on the outside periphery of said shaft, supports a developer on its surface to form thin films thereof and in this state, rotates in contact with or in close vicinity to a latent image preserving body that preserves an electrostatic image on its surface, and thus supplies the developer to the surface of the latent image preserving body so as to visualize an electrostatic image on the surface of the latent image preserving body, said electroconductive elastic layer being composed of a rubber composition comprising (A) butadiene rubber, (B) isoprene rubber in the form of liquid and (C) silicone rubber at a {(A)+(B)}/(C) ratio by weight in the range of 97/3 to 5/95.
Furthermore, the present invention provides a developing apparatus comprising a latent image preserving body capable of preserving an electrostatic image on the surface thereof and a developing roller which is placed so as to rotate in contact with or in close vicinity to the latent image preserving body, which supports a developer on its surface to form thin films thereof and which supplies the developer to the surface of the latent image preserving body so as to visualize an electrostatic image on the surface of the latent image preserving body, wherein the developing roller in the preceding item is used as the developing roller.